Permanent magnet direct current motors are sometimes used for submerged fuel pump applications. These motors typically employ either face-type commutators or cylinder or "barrel"-type commutators. Face-type commutators have planar, circular commutating surfaces disposed in a plane perpendicular to the axis of armature rotation. Barrel-type commutators have arcuate, cylindrical commutating surfaces disposed on the outer surface of a cylinder that is positioned coaxially around the axis of armature rotation. Regardless of their commutating surface configurations, electric motors used in submerged fuel pump applications must be small and compact, have a long life, be able to operate in a corrosive environment, be economical to manufacture and operate and be essentially maintenance-free.
Submerged fuel pump motors must sometimes operate in a fluid fuel medium containing an oxygen compound, such as methyl alcohol and ethyl alcohol. The alcohol increases the conductivity of the fuel and, therefore, the efficiency of an electrochemical reaction that deplates any copper motor components that are exposed to the fuel. For this reason, carbon and carbon compositions are sometimes used to form carbon segments with segmented commutating surfaces for the motors. This is because carbon commutators do not corrode or "deplate", as copper commutators do. Commutators with carbon segments also typically include metallic contact sections that are in electrical contact with the carbon segments and provide a terminal for physically connecting each electrical contact to an armature coil wire.
It is known to form a carbon commutator by first molding and heat treating a moldable carbon compound or machining heat-treated carbon or carbon/graphite stock. Such an arrangement is shown in German Disclosure 3150505.8. A commutator-insulating hub may then be formed to support the metallic substrate. The hub may be molded directly to the metallic substrate either before or after the carbon is bonded to the metallic substrate. Slots are then machined through the carbon article and the metallic substrate to separate the carbon article and substrate into a number of electrically isolated segments. An inner diameter, outer diameter and the commutating surface of the commutator may also need to be machined.
After the completed commutator is assembled to an armature, a clamshell mold may be positioned over the newly assembled commutator-armature in a final overmolding process. An open end of the clam shell mold is made to seal around the commutator in a manner that leaves the commutating surface exposed. Insulator material is then injected into the clam shell mold. Once the insulator material has cured, the clam shell mold is removed. This final overmolding step protects copper armature windings and other corrosion-prone elements from chemically reacting with ambient fluids such as oxygenated fuels. The overmolding also secures wires to reduce potential for stress failures and to maintain a corrected dynamic balance level. Overmolding will also reduce windage losses in the pump.
Where, in manufacturing such a commutator, cuts are machined into or through a metallic substrate, metal chips may be produced. These metal chips can lodge in the slots between segments causing electrical failures. Machining into a metallic substrate can also expose the cut portions of the substrate to the corrosive effects of oxygenated fuels.
Where the carbon and metal substrate portions of a commutator are machined-through to form electrically isolated segments, some type of support structure must be provided to strengthen the commutator and mechanically bind the carbon segments and conductor sections together. Such support structures sometimes require substantial additional axial space for the commutator, which can increase the overall axial length of the armature-commutator assembly and or reduce the size and the quantity of wire wound in the armature.
For some types of electrical-conducting resin-bonded carbon compositions, an insulating surface skin characteristically forms on exterior surfaces of the composition as it cures. This skin forms an impediment to electrical contact between the carbon composition and the metallic conductor sections. Therefore, a carbon commutator using such a composition must provide an electrical path through the insulating surface skin.
One approach to solving these problems is disclosed in U.S. Pat. No. 5,386,167 issued Jan. 31, 1995 to Strobi (the Strobi patent). The Strobi patent shows a carbon disk made up of an electrical-conducting resin-bonded carbon composition. To avoid problems associated with machining into metal substrates, the carbon disk is overmolded onto eight pie-piece-shaped copper segments then radially cut between the segments to form eight electrically isolated carbon segments. A plastic substrate holds the copper segments in position for carbon overmolding and provides mechanical interlock between the carbon segments. However, the plastic substrate increases the axial thickness of the commutator. In addition, the Strobi patent does not provide structures that would provide an electrical path through carbon composition skinning or structures that might otherwise reduce electrical resistance.
What is needed is a carbon-segment commutator that is stronger and provides lower electrical resistance through increased carbon to copper contact within the carbon segments and through any insulating surface skin that might form. What is also needed is a method for manufacturing such a commutator that requires less machining time and provides longer tool life.